1. Field of the Invention
The present invention relates to mobile tree shears of the type using blades for shearing trees and, more specifically, to novel blade structure for such tree shears.
2. Prior Art
Tree shears are known having opposed swinging blades power driven toward each other for shearing a standing tree. Some known tree shears use wedge-shaped blades increasing in thickness in the direction away from their cutting edges, but, as noted in Allen U.S. Pat. No. 3,640,322, preferably thin flat blades of uniform thickness are used so that less power is required to slice through the tree.
Thin flat blades of uniform thickness require reinforcement so that the blades will withstand the force required to shear a tree without bending or breaking. Such reinforcement usually is a thick rib or plate formed on or secured to the margin of a blade remote from its cutting edge so that the rib or plate will not contact a tree being sheared. For example, in some known blades a peripheral reinforcing rib is cast integral with the blade, or a reinforcing plate is welded to the blade. A problem with this type of blade structure is that the entire blade unit must be scrapped if the thin tree-shearing portion of the blade becomes worn beyond repair.
In some known tree shears, thin flat blades of uniform thickness are detachably mounted on separate peripheral reinforcing members or "blade carriers", such as by bolts, so that a blade can be replaced without scrapping the associated blade carrier. For example, in one known tree shear, a thin flat blade is bolted onto the bottom surface of a blade carrier which, in turn, is pivotally mounted on suitable support structure carried by a tractor. A problem with this type of blade structure is that blade-driving force is applied to the blade carrier and transmitted to the blade only by the connecting bolts. Such force can be great enough to shear the bolts and thereby detach the blade from the blade carrier.
Hamilton U.S. Pat. No. 3,527,272 discloses tree shear blade structure including a semicircular blade or "knife" having a straight cutting edge and detachably mounted on a "knife holder" by "studs". About one-half of the arcuate blunt edge portion of the blade is received in an arcuate groove in the holder. A problem with the Hamilton blade structure is that blade-swinging force is applied to the holder rather than directly to the blade and cutting reaction force applied to the blade at any point along its cutting edge other than precisely at the center of the cutting edge tends to slide and rotate the blade relative to the holder. Such sliding rotation is resisted only by the studs. Consequently, as in the blade structure discussed above, in the Hamilton blade structure substantial force is transmitted between the blade and the blade holder only by the studs connecting the blade to the blade holder.
Another problem with known tree shears using a blade detachably mounted on a blade carrier is that the blade-swinging power drive mechanism applies force offset from the plane of the blade. For example, in the known tree shear having a thin flat blade bolted to the bottom of a blade carrier, the power drive mechanism is in the form of a hydraulic jack having its plunger connected to the blade carrier above the blade. Force applied to the blade carrier by the jack is applied in a plane parallel to but above the plane of the blade, and it is in the lower plane of the blade where such force is applied to a tree by the cutting edge of the blade. A substantial moment tending to twist the blade carrier results, increasing the stress on the pivotal mounting for the blade carrier and increasing the stress on the bolts connecting the blade to the blade carrier. This also is a problem with the "Means for Cutting Trees" of Jonsson U.S. Pat. No. 3,540,501 in which a cupped blade is bolted to the underside of a blade carrier and blade-swinging force is applied to the blade carrier above the blade.